We report the theory and experiment of how an ultrathin (<80 nm) layer of vanadium dioxide (VO2) can be used to control and adjust the polarization state of light. The refractive index of vanadium dioxide undergoes large changes when the material makes a phase transition from semiconductor to metal at a temperature of 68 °C. In a thin film, this results in optical phase shifts that are different for s- and p-polarizations in reflection or transmission. We investigate the conditions under which the polarization state would changes between linear or circular or between linear polarizations oriented differently during the material’s phase transition. The effect is demonstrated from 600 nm to 1600 nm and optical devices are proposed based on experimental data on refractive indices with temperature.